OTA testing of MIMO Wi-Fi and LTE radios

Modern radio technologies such as the IEEE 802.11 and 3rd generation partnership project (3GPP) long term evolution (LTE) depend on multiple input multiple output (MIMO) techniques to increase the radio transmission range and speed. MIMO algorithms use multiple synchronised radios (up to 4 for 802.11n and LTE; up to 8 for the emerging 802.11ac) to adapt to continuously changing conditions in the wireless channel.

These techniques include TX and RX diversity to add robustness to the communications when channel conditions are challenging (e.g. low SNR or high multi-path); spatial multiplexing to increase throughput by sending multiple simultaneous streams when channel conditions are favourable; beam forming to extend range or to enable multiple users to share the wireless channel; and MU-MIMO (multi-user MIMO) to enable multiple stations to transmit simultaneously in the same frequency channel by focusing the antenna pattern.

Modern MIMO radios sense the conditions in the channel on a packet-by-packet basis and make instantaneous decisions on which of the above techniques to employ. Testing of these radios requires new generation over-the-air (OTA) technology. In the past, radios could be tested in a conducted environment whereby antennas are disconnected and coaxial cabling connected instead of antennas to guide the signal to controlled test circuitry (figure 1).

Figure 2: Measurements obtained in the uncontrolled test environment (top) can be highly variable. Measurements obtained in a controlled environment (bottom), which can be conducted or controlled OTA, are stable and repeatable. Accuracy of the measurements is a function of proper calibration.

While the conducted test environment is considered controlled, OTA testing can be performed under controlled or uncontrolled conditions. Uncontrolled OTA test methods include using a typical house or outdoor setting to measure throughput and range of the devices. Controlled OTA testing is typically performed in an anechoic chamber. Uncontrolled environment results in measurements that can vary over time. Controlled environment, either conducted or OTA, when properly implemented produces repeatable measurements as shown in figure 2.